ЯДЕРНАЯ ФИЗИКА, 2012, том 75, № 5, с. 623-627
= ЭЛЕМЕНТАРНЫЕ ЧАСТИЦЫ И ПОЛЯ
FIRST RESULTS FROM THE ALICE EXPERIMENT
©2012 I. Belikov* (for the ALICE Collaboration)
IPHC, Université; de Strasbourg, CNRS-IN2P3, France Received March 31, 2011
The results from first series of measurements performed by the ALICE experiment at the LHC are presented. These measurements include the charged-particle pseudorapidity densities, multiplicity distributions and transverse momentum spectra obtained by analyzing the data collected in 2009 and 2Ol0 in proton—proton collisions at three different center-of-mass energies of 0.9, 2.36, and 7 TeV. The results are compared to previous proton—antiproton data and to model predictions.
1. INTRODUCTION
The energy density expected to be reached in PbPb collisions at the LHC will be of the order of a few tens of GeV/fm3. Under these conditions, a deconfined state of quarks and gluons, the quark— gluon plasma, is expected to be formed. A Large Ion Collider Experiment (ALICE) [1] at CERN is a general-purpose heavy-ion experiment designed to study the physics of this new state of matter. However, these studies cannot be effectively performed without having a reliable "hadronic reference". All the measurements that will be done by ALICE in heavy-ion collisions will have to be compared with the corresponding, properly scaled, proton—proton results. Also, it is important to check if the models that successfully described the particle production in "elementary" collisions at lower energies still do so when extrapolated to the LHC energy domain. Finally, one might expect something completely new (like some kind of collective effects at the partonic level) to happen in pp collisions at these new LHC energies. For this, we would need to study all the observables as the function of multiplicity. Which, in turn, requires good multiplicity measurements.
The description of the ALICE detectors can be found in [1]. As for the first pp runs in 2009 and 2010, the most important detectors are the Inner Tracking System (ITS), the Time Projection Chamber (TPC), the Time-Of-Flight detector (TOF) (all covering the pseudo-rapidity range \n\ < 0.9), and the muon spectrometer (MUON) (with the pseudorapidity coverage —4 <n < -2.5). All these detectors are fully installed and operational.
The triggering for the first collisions is essentially minimum bias. At least one charged particle is required in about 8 units of pseudorapidity covered
* E-mail: iouri.belikov@in2p3.fr
by the two innermost pixel layers of the ITS (with the pseudorapidity windows of \n\ < 2.0 and \n\ < < 1.4, respectively) and the two scintillating rings of the V0A and the V0C detectors (with the coverage of 2.8 <n < 5.1 and —3.7 <n < —1.7). A special single-muon trigger is also implemented for triggering the muon spectrometer, and it is read out in coincidence with the general minimum-bias trigger. The coincidence with the beam pickup counters, which can be done in several logical combinations, allows for accepting special control triggers used for the background estimations.
We have taken data at three energies: 0.9, 2.36, and 7 TeV. At 0.9 and 2.36 TeV we consider two event classes: inelastic (INEL) and non-single-diffractive (NSD). The corresponding corrections for the triggering efficiency are done by tuning the MC event class fractions to match the UA5 [2], E710 [3], and CDF [4] data, and using PYTHIA 6.4.14 and 6.4.21 [5, 6], tune D6T [7], and PHOJET 1.12 [8] Monte Carlo models to assess the effects of different kinematics. At 7 TeV, we use the hadron-level event class definition, requiring at least one charged particle in a pseudorapidity window \n\ < 1.
2. THE FIRST PHYSICS RESULTS
2.1. Charged-Particle Pseudorapidity Density and Multiplicity Distributions
The first multiplicity measurements in ALICE were done with the two inner most pixel layers of the ITS. ALICE has published the charged-particle pseudorapidity (dNch/dn) densities and multiplicity distributions obtained for the INEL and NSD events in pp collisions at 0.9, 2.36, and 7 TeV [9—11]. The results were found to be consistent with UA5 pp data at 0.9 TeV [12, 13], and also with CMS pp data at 0.9 and 2.36 TeV [14] (for the NSD events).
ALICE preliminary pp, INEL, Js = 0.9 TeV
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—- PYTHIA D6T (109)
......... PYTHIA ATLAS-CSC (306)
PYTHIA Perugia 0 (320)
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Fig. 1. The transverse-momentum spectrum for the K% measured in INEL pp collisions and compared with different PYTHIA tunes and with the PHOJET.
The obtained multiplicity distributions fit well a single negative binomial distribution, and the distribution for the INEL events tends to have more events with low multiplicity than the NSD events do. The dependence of dN^/dr) on the collision energy sfs is well described by the power-law function (^s0-1) [11]. This increase with ^fs is systematically bigger than predicted by several PYTHIA tunes and PHOJET.
2.2. Mid-Rapidity Anti-Proton to Proton Ratio
This measurement provides an important piece of information for the theoretical discussion over the question: what carries the baryon number? The models postulating that the baryon number is carried by valence di-quarks, predict the p/p ratio should be 1 at the LHC energies. Other models involve special configurations of gluonic field, the so-called "string junctions", that may also be related to the baryon number (see, for example, the references in [15]). In these models, the p/p ratio does not reach 1 even at the LHC energies.
ALICE has performed this measurement and has added two important points to the p/p excitation function, at 0.9 and 7 TeV. At 7 TeV, the measured value of p/p is 0.990 ± 0.006 ± 0.014 (the first error is statistical, the second is systematic), which is consistent with 1. The ratio has also been studied as the function of the transverse momentum. It has been found that the models with string junctions systematically underestimate the data, especially at 0.9 TeV [15].
Altogether, these results put a strong constraint on the association between the string junctions and the baryon number.
2.3. Bose—Einstein Correlations with ChargedPions
The Bose—Einstein correlations of identical pions are a well known tool to evaluate the size and the space—time evolution of the emitting system. ALICE has measured the corresponding correlation function in pp collisions at 0.9 TeV as the function of qinv = = |p1 _ P21 (p1 and p2 being the momenta of the pions in a pair) [16].
The radius of the emitting system grows with the event multiplicity, consistently with the measurements by other experiments. At the same time, it stays approximately constant as the function of the absolute value of the difference between the transverse momenta of the pions in a pair, kT = = |p1)T _ P2,T 1/2, which is different from the previous observations. We also report a significant systematic uncertainty inevitably affecting all these measurements. This systematic uncertainty comes from the assumption about the shape of the "base line", which is, likely, not to be flat in pp, as it is often assumed.
2.4. Particle Production and Transverse-Momentum Spectra
The charged-particle transverse-momentum spectrum measured by ALICE in the pseudo-rapidity window Inl < 0.8 [17] fits well the modified Hagedorn function. At the transverse momenta pT > 3 GeV/c,
FIRST RESULTS FROM THE ALICE EXPERIMENT
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Fig. 2. The transverse-momentum spectrum of the A0 measured in INEL pp collisions and compared with different PYTHIA tunes and with the PHOJET.
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Fig. 3. The transverse-momentum spectrum of J measured by the ALICE MUON spectrometer in the ^T decay channel.
the shape of the spectrum is somewhat better described by the power-law function. The comparison to the PHOJET and different PYTHIA tunes shows that none of these Monte Carlo models gives a reasonable description of the data.
We also compare this spectrum with the ones measured by other experiments (UA1, ATLAS, and CMS, see the references in [17]). ALICE's spectrum is harder. This has been understood as the consequence of the narrower pseudorapidity window that is used by ALICE.
A rather strong test of the available models can be performed by looking at the correlation between the
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Mean = 145.43 ± 0.03 MeV/c2 Sigma = 620 ± 34 keV/c2
0.135 0.140 0.145 0.150 0.155 0.160 M(Knn) - M(Kn), GeV/c2
Fig. 4. The invariant mass spectrum of charged D* mesons identified by the ALICE ITS, TPC, and TOF detectors in the D0n decay channel.
mean pT and the multiplicity. It turns out that none of the used Monte Carlo models is able to simultaneously reproduce the measured correlation [17] and the multiplicity distributions reported in [10].
Up to the moment of presenting these results (August 2010), ALICE has also obtained the transverse-momentum spectra of identified particles (^±, K±, p±, 4>, K0, A0, A0, and S±) in pp collisions at 900 GeV. The charged particle identification is done by combining the dE/dx measurements in the ITS
and TPC with the TOF information provided by the TOF detector. The 0 mesons are reconstructed in the 0 ^ K+K- channel using the invariant mass method. The K°s and the strange baryons are identified by their V0 and cascade decay topolo
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